The present invention relates to drive circuits for powering AC motors, and is especially directed to a speed control circuit that generates bipolar pulses to power a motor, such as an AC induction motor. The invention is especially directed to an arrangement for controlling the air flow into an air conditioned space, with blower speed being controlled in accordance with air conditioning load, in a manner to conserve electrical energy when the blower is operated at operating speed or lower speed, and in a manner that avoids insulation breakdown to the blower motor.
In HVAC systems, such as home air conditioning systems, it is often necessary to change the fan speed or blower speed to control the amount of air flow through the system evaporator coil. Cold, dry air is considerably heavier than warm moist air, and so during initial operation the blower has to operate at high speed to pump conditioned air, especially to higher floors. Then, when the comfort space or living space has cooled down, the fan speed is reduced to avoid blowing cold air directly on human occupants. Also, where sensible cooling is needed, rather than latent cooling (dehumidification) the blower is operated at higher speed to increase air flow. Correspondingly, if dehumidification is required more than sensible cooling, the air flow rate should be reduced, requiring a slower blower speed. Other air conditioning load considerations can also create air flow requirements to govern blower speed, such as heat and humidity requirements for indoor plants, or preservation of expensive art works or musical instruments.
Various approaches to control of a blower motor are discussed in the art. U.S. Pat. No. 4,978,896 to Shah discusses the need to maintain rotational speed over a range of static pressures to keep air flow at a desired level. A microprocessor generates a pulse width modulated (PWM) signal for motor drive power, and uses a motor current sensor as a feedback. U.S. Pat. No. 4,734,012 to Dob et al. discusses a speed control circuit for a blower motor, in which AC voltage is rectified and regulated responsive to an ambient temperature, and then an optoisolator and a triac control the AC through the motor. U.S. Pat. No. 4,879,503 to Aoki et al. discusses a blower motor control for an air conditioner that controls the blower motor rotational speed by comparing it with a target speed. Here, a microcomputer looks for zero crossings of the motor drive, and emits pulses to actuate a triac in series with the motor armature. Another approach to fan or blower speed control is discussed in U.S. Pat. No. 4,722,669 to Kundert, where a DC motor is energized in a unipolar pulsed DC mode.
A recent approach to motor control, which was designed to create control over motor speed over a wide power range, e.g., from several watts to several kilowatts, has been an adjustable speed drive (ASD) employing a pulse-controlled inverter. In these ASD's the incoming AC power is rectified to produce a constant DC level, and that is converted to an AC drive wave using pulse-width modulation (PWM). These ASD's overcome the shortcomings of operating induction motors directly on line voltage, and satisfy many of the requirements for speed control. Unfortunately, the use of PWM can lead to other problems, including winding insulation failure in the motor armature.
In the PWM ASD drive circuit, the DC voltage is usually gated or switched on and off several times per half-cycle to create a drive wave of the appropriate power characteristic. The many pulses for each half wave are intended to regenerate a sinusoidal wave, using Fourier transform principles. This means that there are many very short pulses at the DC rail voltage, each with a very short rise time. Typically, where an IGBT bridge converter is used, the switching frequencies can be on the order of 20 kHz or more, with rise times of between 20 and 100 ns. At the same time, the insulation in the motor's armature windings have a limited insulation strength, and the high frequency pulses that occur in these ASDs can overstress the interturn winding insulation. In many motors, where the windings are randomly wound, the interturn high frequency voltages can be even more exaggerated. This problem is discussed in Kaufhold et al., Failure Mechanism of the Interturn Insulation of Low Voltage Electric Machines Fed by Pulse-Controlled Inverters, IEEE Electrical Insulation Magazine, September/October 1996, pp. 9 to 16. The problem of using high switching frequencies for variable speed PWM drives of this type is also discussed in Lowery, General Purpose Variable-Speed Drive and Motor Application Considerations, ASHRAE Transactions 1998, vol. 104, pt. 2. Because of the finite rise times of the switching pulses, the switching transistors of the inverter spend a finite time in their active range when the transistor transitions between cut off and conduction. In that region, energy is converted to waste heat. This problem becomes exaggerated where there are multiple high frequency pulses for each half cycle of the drive wave.